1. The Field of the Invention
The present invention relates to differential signal decoders wherein an output is provided representative of the difference between two input signals according to a predetermined scale factor. It particularly relates to differential signal decoders wherein the output is scaled such that the sum of the input signals is effectively a predetermined constant value. In greatest particularity the present invention relates to decoders for recorded servo signals on moving data storage media operable to give an output signal indicative of the displacement of a transducer from being centrally disposed over the servo track by taking the difference between two position-indicating signals recorded on the medium.
While the present invention is hereafter described in relation to the decoding of servo signals recovered from a moving data storage medium, it is to be appreciated that this application is intended to be merely illustrative of one field of use of the invention and does not constitute a limitation to its utility.
2. The Prior Art
Numerous schemes exist for recording servo tracks on a moving data storage medium. The medium is popularly disc or tape and the servo tracks are used to indicate the displacement of a signal-recovering transducer from being centrally disposed thereover by decoding the servo signal recovered from the medium by the transducer. The output of the servo signal decoder can then be used either to set up some initial position of the transducer relative to the medium or continously during the operation of the data storage equipment to provide a position feedback signal for a transducer-positioning servomechanism to control the position of the transducer relative to the medium.
The nature of the servo signals involves first and second signal components which increase and decrease respectively as the transducer moves away from central disposition over the servo track in a first direction and decrease and increase respectively as the transducer moves away from central disposition over the servo track in a second direction. The difference between the first and second components is used to indicate by magnitude and sense the magnitude and direction of the displacement of the transducer away from being centrally disposed over the servo track. It is a problem that the amplitude of the signals recovered from the medium is uncertain and that, in order for the output of a servo signal decoder to be consistent in its magnitude, scaling means must be included for compensating for or for eliminating the effects of collective amplitude variation by the first and second signal components.
In one scheme the output of the transducer is provided as the input to a gain-controlled amplifier. The output of the gain-controlled amplifier is provided as the input to a measuring circuit which measures the amplitudes of the first and second signal components, sums them and subtracts the sum from a reference level. The result of the substraction is used as the gain controlling input to the gain controlled amplifier which thereby acts to maintain the sum of the first and second signal components equal to the reference level and so stabilize the scaling of the difference therebetween. This scheme has the disadvantage that the speed of the gain-controlled amplifier is slow and requires a considerable period of servo signals being present for the described, stabilized condition to be established. It is the trend that servo information is included in short segments among data. Such a slow scheme would not work on the short blocks of servo signals.
In another scheme the magnitudes of the first and second components are stored on first and second capacitors respectively which are discharged with the same time constant, the sum of the voltages on the capacitors being compared against a reference and a sample and hold circuit being triggered to acquire and hold the difference between the voltages on the capacitors at the instant the sum of the voltages equals the reference. This requires the inclusion of a controller for supervising the above actions and also requires that the time constants of the discharging of the two capacitors are closely matched, which can be a difficult task given the variation in values encountered among manufactured capacitors of nominally the same value.
One very common type of servo track is the so-called tribit track described by Meuller in U.S. Pat. No. 3,691,543 wherein two contiguous adjacent subtracks are recorded on a magnetic data storage disc. Both subtracks share a common magnetic flux transition of a first sense. The first subtrack thereafter has its own individual flux transition of a second sense and thereafter the second subtrack has its own flux transition of the second sense. When read back by a replay head, the common transition of the first sense produces a first timing pulse of a first polarity followed by a first position pulse of the opposite polarity caused by the individual transition of the first subtrack. Thereafter the individual transition of the second subtrack causes a second position pulse of the same polarity as the first position pulse. The relative amplitudes of the first and second position pulses depends upon the proportion of the width of the head disposed over each subtrack and the difference in their amplitudes is therefore indicative of the displacement of the head from being equally disposed over each subtrack.
Whereas the invention is hereafter described with reference to use with so-called tribit signals, it is to be appreciated that this is merely illustrative of one manner of use in one of many possible fields of application.
It is therefore desirable to provide a signal decoder which is operable at high speed without the necessity for complicated control sequences for automatically providing an output proportional to the difference between two input signals scaled as if the sum of the two input signals were equal to a predetermined value regardlessly of their actual sum. It is further desirable that means be provided for operating such a decoder for decoding servo signals recovered from a moving medium by a transducer. It is yet further a desirable feature that those signals be so-called tribit signals.